= Executing Gradle builds on Jenkins

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Building Gradle projects doesn't stop with the developer's machine.
https://en.wikipedia.org/wiki/Continuous_integration[Continuous Integration] (CI) has been a long-established practice for running a build for every single change committed to version control to tighten the feedback loop.

In this guide, we'll discuss how to configure link:https://jenkins.io/[Jenkins] for a typical Gradle project.

== What you'll need

* A text editor
* A command prompt
* The Java Development Kit (JDK), version 1.7 or higher
* A https://gradle.org/install[Gradle distribution], version {gradle-version} or better
* A Jenkins installation (setup steps explained in this post)

== Setup a typical project

As example, this guide is going to focus on a Java-based project.
More specifically, a Gradle plugin written in Java and tested with Spock.
First, we'll get the project set up on your local machine before covering the same steps on CI.

Just follow these steps:

=== Clone https://github.com/gradle/gradle-site-plugin[Gradle Site Plugin] repository

[listing.terminal.sample-command]
----
$ git clone https://github.com/gradle/gradle-site-plugin.git
Cloning into 'gradle-site-plugin'...
$ cd gradle-site-plugin
----

=== Build the project

As a developer of a Java project, you'll typical want to compile the source code, run the tests and assemble the JAR artifact. That's no different for Gradle plugins. The following command achieves exactly that:

[listing.terminal.sample-command]
----
$ ./gradlew build

BUILD SUCCESSFUL
14 actionable tasks: 14 executed
----

The project provides the link:{user-manual}gradle_wrapper.html[Gradle Wrapper] as part of the repository.
It is a recommended practice for any Gradle project as it enables your project to built on CI without having to install the Gradle runtime.

=== Build scan integration

The sample project is equipped with support for generating https://scans.gradle.com/[build scans].
Running the build with the command line option `--scan` renders a link in the console.

[listing.terminal.sample-command]
----
$ ./gradlew build --scan

Publishing build scan...
https://gradle.com/s/7mtynxxmesdio
----

The following section will describe how to build the project with the help of Jenkins.

== Setup Jenkins

Jenkins is one of the most prominent players in the field.
In the course of this section, you'll learn how to set up Jenkins, configure a job to pull the source code from GitHub and run the Gradle build.

=== Install and start Jenkins

On the https://jenkins.io/download/[Jenkins website] you can pick from a variety of distributions.
This post uses the runnable WAR file.
A simple Java command brings up the Jenkins server.

// TODO: this block will start a background process not handled by the content testing
[listing.terminal]
----
$ wget https://mirrors.jenkins.io/war-stable/latest/jenkins.war
$ java -jar jenkins.war
----

In the browser, navigate to `localhost` with port `8080` to render the Jenkins dashboard.
You will be asked to set up an new administration user and which plugins to install.

=== Installation of plugins

Confirm to install the recommended plugins when starting Jenkins for the first time.
Under "Manage Jenkins > Manage Plugins" ensure that you have the following two plugins installed.

* https://plugins.jenkins.io/git[Git plugin]
* https://plugins.jenkins.io/gradle[Gradle plugin]

Next, we can set up the job for building the project.

== Create a Jenkins job

Setting up a new Gradle job can be achieved with just a couple of clicks.
From the left navigation bar select "New Item > Freestyle project".
Enter a new name for the project. We'll pick "gradle-site-plugin" for the project.

Select the radio button "Git" in the section "Source Code Management".
Enter the URL of the GitHub repository: `https://github.com/gradle/gradle-site-plugin.git`.

image::ci-systems/jenkins-scm.png[]

Furthermore, create a "Build step" in the section "Build" by selecting "Invoke Gradle build script".
As mentioned before, we'll want to use the Wrapper to execute the build. In the "Tasks" input box enter the `build` and use the "Switches" `--scan -s` to generate a build scan and render a stack trace in case of a build failure.

image::ci-systems/jenkins-build-step.png[]

=== Execute the job

Save the configuration of job and execute an initial build by triggering the "Build Now" button.
The build should finish successfully and render a "Gradle Build Scan" icon that brings you directly to the link:https://scans.gradle.com[build scan] for the given build.

image::ci-systems/jenkins-build-scan.png[align=center]

There are various options to trigger Jenkins builds continuously: from polling the repository periodically, to building on a set schedule, or via callback URL.

== Further reading

You can learn more about advanced Jenkins usage through these resources:

* https://jenkins.io/doc/book/using/using-credentials/[Using credentials with Jenkins]
* https://jenkins.io/solutions/pipeline/[Pipeline as code with Jenkins]
* https://bmuschko.com/blog/jenkins-build-pipeline/[Modelling a Continuous Deployment pipeline for a Spring Boot application]

== Summary

Executing Gradle builds on CI can be set up and configured with just a handful of steps.
The benefit of receiving fast feedback clearly speaks for itself.
If you are not using Jenkins, no problem, many CI products tightly integrate with Gradle as a first-class citizen.

